There are known thermoelectric generators (TEG) which are capable of providing low voltage electrical energy, on the order of several millivolts, for charging a battery of a portable electronic device by means of an inductive voltage boost converter. An electrical diagram of such a device is partially shown in FIG. 1.
Electronic device 2 includes a thermoelectric generator 4, represented by a voltage source VTEG and an internal voltage RTEG, a battery 6 and a voltage booster 8. This voltage booster is formed by a buffer capacitor C0 at input, an inductor L and an integrated circuit 10 arranged between the inductor output terminal VLX and the positive battery terminal VBAT. It will be noted that the buffer capacitor and the inductor are discrete elements. The integrated circuit includes a diode D1 or a plurality of diodes D1, D2 arranged in parallel between terminal VLX and the positive battery terminal, a smoothing capacitor C1 and a switch TM formed by an MOS transistor. This switch receives a control signal S1 from a control unit (not shown) to control its actuation. When generator 4 supplies sufficient energy, the battery is charged by periodically switching switch TM between its closed position (transistor is conductive) and its open position (transistor is non-conductive).
For thermoelectric generators which capture thermal energy from their environment in the presence of a small temperature difference, the voltage supplied varies with this temperature difference so that the voltage can drop below a minimum value at which the battery charging system output becomes zero. Thus, below this minimum value, the voltage booster must be deactivated to avoid discharging the battery. In order to do this, it is necessary to measure periodically the voltage at the voltage booster input. This poses a technical problem when the voltage Vin supplied by the thermoelectric generator in normal operation is very low, on the order of several millivolts (mV), and when said minimum value is, for example, approximately equal to 1 mV. This situation occurs, for example, with a watch worn on a user's wrist incorporating a conventional thermoelectric generator which provides a voltage of between 5 mV and 10 mV per degree (5-10 mV/K). The usable temperature difference is on the order of one Kelvin when the watch is worn on the user's wrist. However, when the watch is removed from the user's wrist and stored, for example, in a box, the voltage supplied becomes zero and it is essential to deactivate the energy collection system which uses energy and thus discharges the battery.
The technical problem arises from the difficulty in measuring such small voltages or corresponding small currents. Indeed, the prior art electronic circuits for measuring such small voltages are complex and sensitive, and therefore expensive and difficult to implement.